I've seen some get slack in them after like 100,000 miles but I've never heard of any breaking. Those double chains are strong as hell. It would be a good idea to replace them if you have more than 10,000 miles on them and the timing chain cover is off.
A forum community dedicated to Ford Mustang owners and enthusiasts. Come join the discussion about performance, builds, modifications, reviews, engine swaps, classifieds, troubleshooting, maintenance, and more!